Re: Fix calloc.m4 test

[Bruno Haible]

On 2020-05-23 I wrote:
> As you can see:
> 
>   1) clang has eliminated the calloc() and free() calls from the program.
>      Apparently it "knows" how to optimize matching calloc - free pairs.
> 
>   2) It has estimated that the second call would return a non-NULL pointer,
>      although the address space does not allow this.
>      Reported at <https://bugs.llvm.org/show_bug.cgi?id=37304>. But some
>      people claim it is not a bug. Paul, can you please help with ISO C
>      citations?
> 
> This patch provides a workaround.
> 
> 
> 2020-05-23  Bruno Haible  <bruno@clisp.org>
> 
>       calloc-gnu: Avoid wrong configure results with clang.
>       * m4/calloc.m4 (_AC_FUNC_CALLOC_IF): Mark the pointer variable as
>       'volatile', to defeat compiler optimizations.

The same problem occurs also in the test suite. The continuous integration [1]
detected it.

[1] https://gitlab.com/gnulib/gnulib-ci/-/pipelines/153500376


2020-06-06  Bruno Haible  <bruno@clisp.org>

        calloc-gnu tests: Avoid a test failure with clang.
        * tests/test-calloc-gnu.c (main): Mark the pointer variable as
        'volatile', to defeat compiler optimizations.

diff --git a/tests/test-calloc-gnu.c b/tests/test-calloc-gnu.c
index c13ba47..2ae3925 100644
--- a/tests/test-calloc-gnu.c
+++ b/tests/test-calloc-gnu.c
@@ -36,20 +36,26 @@ int
 main ()
 {
   /* Check that calloc (0, 0) is not a NULL pointer.  */
-  void *p = calloc (0, 0);
-  if (p == NULL)
-    return 1;
-  free (p);
+  {
+    void * volatile p = calloc (0, 0);
+    if (p == NULL)
+      return 1;
+    free (p);
+  }
 
   /* Check that calloc fails when requested to allocate a block of memory
      larger than SIZE_MAX bytes.
-     We use eight (), not 8, to avoid a compiler warning from GCC 7.  */
-  p = calloc ((size_t) -1 / 8 + 1, eight ());
-  if (p != NULL)
-    {
-      free (p);
-      return 1;
-    }
+     We use eight (), not 8, to avoid a compiler warning from GCC 7.
+     'volatile' is needed to defeat an incorrect optimization by clang 10,
+     see <https://bugs.llvm.org/show_bug.cgi?id=46055>.  */
+  {
+    void * volatile p = calloc ((size_t) -1 / 8 + 1, eight ());
+    if (p != NULL)
+      {
+        free (p);
+        return 2;
+      }
+  }
 
   return 0;
 }